Encyclopedia of Espionage, Intelligence, and Security

Nuclear Reactors

Nuclear reactors are complex devices in which fissionable elements such as
uranium, thorium, or plutonium are made to undergo a sustainable nuclear
chain reaction.

Nuclear reactor at the Bhabha Atomic Research Center in Bombay,
photographed in 1997, near the site of a unit that extracted plutonuim
for use in India's 1974 nuclear tests.

AP/WIDE WORLD PHOTOS

.

This chain reaction releases energy in the form of radiation that (a)
sustains the chain reaction; (b) transmutes (i.e., alters the nuclear
characteristics of) nearby atoms, including the nuclear fuel itself; and
(c) may be harvested as heat. Transmutation in nuclear reactors of the
common but weakly fissionable nuclide uranium-238 (
238
U) into plutonium-239 (
239
Pu) is an important source of explosive material for nuclear weapons, and
heat from nuclear reactors is used to generate approximately 16 percent of
the world's electricity and to propel submarines, aircraft
carriers, and some other military vessels. Nuclear reactors have also been
used on satellites and proposed as power sources for locomotives,
aircraft, and rockets.

How a nuclear reactor works.
A nuclear reactor exploits the innate instability of some atoms—in
general, those that have a large atomic number or that contain an
imbalance of protons and neutrons—which break apart (fission) at
random times, releasing photons, neutrons, electrons, and alpha particles.
For some nuclides (atomic species having a specific number of protons and
neutrons in the nucleus), the average wait until a given atom
spontaneously fissions is shorter. When enough atoms of such an unstable
isotope are packed close together, the neutrons released by fissioning
atoms are more likely to strike the
nuclei of nearby unstable atoms. These may fission at once, releasing
still more neutrons, which may trigger still other fission events, and so
forth. This is the chain reaction on which nuclear reactors and
fission-type nuclear bombs depend. In a reactor, however, the fission rate
is approximately constant, whereas in a bomb it grows exponentially,
consuming most of the fissionable material in a small fraction of a
second.

To produce a sustained chain reaction rather than a nuclear explosion, a
reactor must not pack its fissionable atoms too closely together. They are
therefore mixed with less-fissionable atoms that do not sustain the chain
reaction. For example, in a reactor utilizing
235
U as its primary fuel, only 3 percent of the fuel is actually
235
U; the rest is mostly
238
U, a much less fissionable isotope of uranium. The higher the ratio of
active fuel atoms to inert atoms in a given fuel mix, the more
"enriched" the fuel is said to be; commercial nuclear power
plant fuel is enriched only 3 to 5 percent
235
U, and so cannot explode. For a fission bomb, 90 percent enrichment would
be typical (although bombs could be made with less-enriched uranium).
Naval nuclear reactors, discussed further below, have used fuels enriched
to between 20 and 93 percent.

Having diluted its active fuel component (e.g.,
235
U), a typical nuclear reactor must compensate by assuring that the
neutrons produced by this diluted fuel can keep the chain reaction going.
This is done, in most reactors, by embedding the fuel as small chunks or
"fuel elements" in a matrix of a material termed a
"moderator." The moderator's function is to slow
(moderate) neutrons emitted by fissioning atoms in the fuel.
Paradoxically, a slow neutron is more likely to trigger fission in a
uranium, plutonium, or thorium nucleus than a fast neutron; a moderator,
by slowing most neutrons before allowing them to strike nuclei, thus
increases the probability that each neutron will contribute to sustaining
the chain reaction. Graphite (a form of pure carbon), water, heavy water
(deuterium dioxide or
2
H
2
), and zirconium hydride can all be used as moderators. Ordinary water is
the most commonly used moderator.

If the chain reaction sustained by a nuclear reactor produces enough heat
to damage the reactor itself, that heat must be carried off constantly by
a gas or liquid as long as the reactor is operating. Once removed from the
reactor, this energy may be ejected into the environment as waste heat or
used, in part, to generate electricity. (Electricity, if generated, is an
intermediate energy form; all the energy generated in a nuclear reactor or
other power plant eventually winds up in the environment as heat.) In the
case of a nuclear-powered rocket, such as the one the U.S. National
Aeronautics and Space Administration (NASA) seeks to develop with its
Project Phoenix, heat is removed from the system by ejected propellant.
Liquid sodium, pressurized water, boiling water, and helium have all been
used as cooling media for nuclear reactors, with pressurized or boiling
water being used by commercial nuclear power plants. Typically, heat
energy removed from the reactor is first turned into kinetic energy by
using hot gas or water vapor to drive turbines (essentially enclosed,
high-speed windmills), then into electrical energy by using the turbines
to turn generators.

Nuclear power sources that do not produce enough heat to melt themselves,
and which therefore require no circulating coolant, have been used on some
space probes and satellites, both U.S. and Russian. Such a power source,
termed a radioactive thermoelectric generator or RTG, consists of a mass
of highly radioactive material, usually plutonium, that radiates enough
heat to allow the generation of a modest but steady flow of electricity
via the thermoelectric effect. The efficiency of an RTG is low but its
reliability is very high.

Reactor byproducts.
The neutron flow inside a reactor bombards, and by bombarding changes,
the nuclei of many atoms in the reactor. The longer a unit of nuclear fuel
remains in a reactor, therefore, the more altered nuclei it contains. Most
of the new atoms formed are radioactive nuclides such as cesium-144 or
ruthenium-106; a significant number are, if
238
U is present, isotopes of plutonium, mostly
239
Pu. (Absorption of one neutron by a
238
U nucleus turns it into a
239
Pu nucleus; absorption of one, two, or three neutrons by a
239
Pu nucleus turns it into a
240
Pu,
241
Pu, or
242
Pu nucleus.) Plutonium is found in nature only in trace amounts, but is
present in all spent nuclear fuel containing
238
U. If it is extracted for use as a reactor fuel or a bomb material, it is
considered a useful by-product of the nuclear reactor; otherwise, it is a
waste product. In either case, plutonium is highly toxic and radioactive,
and remains so for tens of thousands of years unless it is further
transmuted by particle bombardment, as in a particle accelerator, reactor,
or nuclear explosion. Reactors specially designed to turn otherwise inert
238
U into
239
Pu by neutron bombardment are termed fast breeder reactors, and can
produce more nuclear fuel than they consume; however, all nuclear
reactors, whether designed to "breed" or not, produce
plutonium.

This fact has a basic military consequence: Every nation that possesses a
nuclear power plant produces plutonium, which can be used to build atomic
bombs. Plutonium sufficiently pure to be used in a bomb is termed
bomb-grade or weapons-grade plutonium, and the process of extracting
plutonium from irradiated nuclear fuel is termed reprocessing. (The alloy
used in sophisticated nuclear weapons is nearly pure plutonium, but the
U.S. Department of Energy has estimated that an unwieldy bomb could be
made with material that is only 15 to 25 percent plutonium, with
less-unwieldy bombs being possible with more-enriched alloys.) Every
nation that possesses a nuclear reactor and reprocessing capability thus
possesses most of what it needs to build nuclear weapons. Several nations,
including India and Pakistan, have in fact built nuclear weapons using
plutonium reprocessed from "peaceful" nuclear-reactor
programs. A large (100 MW electric) nuclear power plant produces enough
plutonium for several dozen bombs a year.

Besides producing plutonium that can, and sometimes is, extracted to
produce nuclear weapons, every nuclear reactor has the feature that if
bombed, its radioactive contents could be released into the environment,
greatly amplifying the destructive effects of a wartime or terrorist
attack. Nuclear reactors thus have a two-edged aspect: as producers,
potentially, of weapons for use
against
an enemy, and as weapons, if attacked,
for
an enemy.

Naval nuclear reactors.
The primary military use of nuclear reactors, apart from the production
of material for nuclear weapons, is the propulsion of naval vessels.
Nuclear power sources enable naval vessels to remain at sea for long
periods without refueling; modern replacement cores for aircraft carriers
are designed to last at least 50 years without refueling, while those for
submarines are designed to last 30 to 40 years. In the case of submarines,
nuclear power also makes it possible to remain submerged for months at a
time without having to surface for oxygen. Furthermore, reactors have the
general design advantage of high power density, that is, they provide high
power output while consuming relatively little shipboard space. A large
nuclear-powered vessel may be propelled by more than one reactor; the U.S.
aircraft carrier USS
Enterprise
, launched in 1960, is powered by eight reactors. Britain, France, China,
and Russia (formerly the Soviet Union) have also built nuclear-powered
submarines and other vessels.

Although the design details of the nuclear reactors used on submarines and
aircraft carriers are secret, they are known to differ in several ways
from the large land-based reactors typically used for generating
electricity. The primary difference is that in order to achieve high power
density, naval reactors use more-highly-enriched fuel. Older designs used
uranium enriched to at least 93 percent
235
U; later Western reactors have used uranium enriched to only 20 to 25
percent, while Russian reactors have used fuels enriched to up to 45
percent. Small quantities of ex-Soviet submarine fuel have appeared on the
global black market; larger quantities could be used as a bomb material.

The first nuclear-powered vessel, was a U.S. submarine launched in 1955,
the USS
Nautilus
. Only three civil vessels (one U.S.-made, one German, and one Japanese)
have ever been propelled by nuclear power; all proved too expensive to
operate. About 160 nuclear-powered ships, mostly military, are presently
at sea; at the peak of the Cold War, there were approximately 250.